linux-kernel - Re: [PATCH v3 1/3] hugetlbfs: stop setting VM_DONTDUMP in initializing vma(VM

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Date:	Fri, 05 Apr 2013 14:32:08 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kosaki.motohiro@...il.com
Subject: Re: [PATCH v3 1/3] hugetlbfs: stop setting VM_DONTDUMP in initializing
 vma(VM_HUGETLB)

(4/3/13 2:35 PM), Naoya Horiguchi wrote:
> Currently we fail to include any data on hugepages into coredump,
> because VM_DONTDUMP is set on hugetlbfs's vma. This behavior was recently
> introduced by commit 314e51b98 "mm: kill vma flag VM_RESERVED and
> mm->reserved_vm counter". This looks to me a serious regression,
> so let's fix it.

I don't think this is enough explanations. Let's explain the code meaning
time to time order.

First, we had no madvice(DONTDUMP) nor coredump_filter(HUGETLB). then hugetlb
pages were never dumped.

Second, I added coredump_filter(HUGETLB). and then vm_dump_size became..

vm_dump_size()
{
	/* Hugetlb memory check */
	if (vma->vm_flags & VM_HUGETLB) {
		..
		goto whole;
	}
	if (vma->vm_flags & VM_RESERVED)
		return 0;

The point is, hugetlb was checked before VM_RESERVED. i.e. hugetlb core dump ignored
VM_RESERVED. At this time, "if (vma->vm_flags & VM_HUGETLB)" statement don't need
return 0 because VM_RESERVED prevented to go into the subsequent flag checks.

Third, Jason added madvise(DONTDUMP). then vm_dump_size became...

vm_dump_size()
{
       if (vma->vm_flags & VM_NODUMP)
               return 0;

	/* Hugetlb memory check */
	if (vma->vm_flags & VM_HUGETLB) {
		..
		goto whole;
	}
	if (vma->vm_flags & VM_RESERVED)
		return 0;

Look, VM_NODUMP and VM_RESERVED had similar and different meanings at this time.

Finally, Konstantin removed VM_RESERVED and hugetlb coredump behavior
has been changed.

Thus, patch [1/3] and [2/3] should be marked [stable for v3.6 or later].

Anyway, this patch is correct. Thank you!

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>

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